Mixing labels or mixing tags are used in Ultra High Frequency (“UHF”) electronic article surveillance (“EAS”) interrogation systems and are based on a frequency mixing principle. Typically, mixing tags include a diode attached to a dipole antenna. The tag is tuned at a specific microwave frequency fuhf), such as for example 915 MHz. The frequency range can be chosen from hundreds of Megahertz up to several Gigahertz by adjusting the antenna's dipole length and the diode's junction capacitance. The lower the operating microwave frequency, the longer the dipole length required, and the larger the resultant capacitance.
However, there are inherent limitations with deactivation devices when having to deactivate tags having diodes. For example, U.S. Pat. Nos. 4,318,090 and 4,574,274 provide UHF mixing tags that use diode non-linear elements and means for direct contact or non-direct contact but with limited range. The breakdown characteristics of the diode requires that a substantial current be driven through diode in order to achieve deactivation, thus resulting in the direct contact with the tag in order to supply sufficient electrical energy to the diode to cause it to be destroyed, thereby deactivating the tag. This results in an impractical deactivation system since it is not always possible or economically feasible to be limited to this type of “contact” deactivation. Thus, tag designs of this type are ineffective in situations where deactivation of the tag takes place from a distance, i.e., where the deactivation device is not in contact with the tag. Other prior art deactivation systems (such as the system disclosed in U.S. Pat. No. 5,608,379) have attempted to avoid this problem by adding switches and other hardware devices to the deactivation system. This proves to be costly and cumbersome and results in relatively low deactivation distances for a considerably large magnetic field source.
Neither of the aforementioned attempted solutions solves the problem of how to effectively deactivate combination EAS tags at a substantial distance without the need for the deactivation device to be in direct contact with the EAS/RFID tag and without the need to provide additional deactivation elements to the EAS tag. The inherent characteristics of diodes with their predictable non-linear behavior render EAS deactivation systems that utilize these types of tags ineffective when it comes to deactivating tags from a distance.
FIG. 1 illustrates a prior art design of a mixing marker 2 or mixing tag often used in electronic article surveillance (“EAS”) systems. Mixing markers of this type are inherently deficient in tag deactivation systems due to the amount of energy needed to destroy the diode, which is typically designed to be robust and rectify and control voltages. The nonlinearity of the intrinsic capacitance and relatively low voltage breakdown characteristics of the MOS capacitor are more desirable than the diode in this aspect. In FIG. 1, the diode 4 appears between two parts of the antenna 6. Tag designs of this type are ineffective in situations where deactivation of the tag takes place from a distance, i.e., where the deactivation device is not in contact with the tag.
The inadequate methods of deactivating EAS tags described above can also apply to combination EAS/RFID tags. With the advent of RFID technology, many retailers are considering tagging merchandise (e.g., per item, per case, per pallet) with RFID tags. At the same time, electronic article surveillance (EAS) technology and devices have proven critical to the reduction of theft and so called “shrinkage”. It is envisioned that RFID devices can also provide many of the same advantages known to EAS technology coupled with additional advantages or capabilities such as inventory control, shelf reading, non-line of sight reading, etc.
Therefore, what is needed is a method of deactivating a combination EAS/RFID tag where the combination tag incorporates a non-linear element that exhibits very low level breakdown characteristics so that reliable deactivation at a considerable distance is achieved.